Highlight is recently focused on optical recording media capable of recording information at a high density and erasing the recorded information for overwriting. One typical rewritable (or erasable) optical recording medium is of a phase change type wherein a laser beam is directed to the recording layer to change its crystalline phase whereupon a change of reflectance by the phase change is detected for reproduction of the information. The phase change optical recording media are of great interest since the optical system of the drive unit used for their operation is simple as compared with magneto-optical recording media.
In most phase change recording media, used are chalcogenide materials such as Ge--Te-based materials and Ge--Sb--Te-based materials, which provide a substantial difference in reflectance between crystalline and amorphous phases and have a relatively stable amorphous phase. Apart from those, recently, it has been proposed to use other compounds known as chalcopyrites. Chalcopyrite compounds have been widely investigated as compound semiconductor materials and have been applied to solar batteries and the like. The chalcopyrite compounds are composed of Ib-IIIb-VIb.sub.2 or IIb-IVb-Vb.sub.2 as expressed in terms of the Groups of the Periodic Table and have two stacked diamond structures. The structure of chalcopyrite compounds can be readily determined by X-ray structural analysis and their basic characteristics are described, for example, in Physics, Vol. 8, No. 8 (1987), pp. 441 and Denki Kagaku (Electrochemistry). Vol. 56, No. 4 (1988), pp. 228. Of the chalcopyrite compounds, AgInTe.sub.2 is known usable in the recording layers of optical recording media having a linear velocity of around 7 m/sec or so, by diluting it with Sb or Bi (see Japanese Patent Application Kokai (JP-A) Nos. 3-240590, 3-99884, 3-82593, 3-73384, 4-151286, etc.). In addition to the optical recording media of a phase change type wherein chalcopyrite compounds are used, also known are other optical recording media of a phase change type wherein is formed an AgSbTe.sub.2 phase in the recording layers being crystallized, such as those disclosed in JP-A Nos. 4-267192, 4-232779 and 6-166268.
For recording information on such a phase change optical recording medium, a high-power (recording-power) laser beam is irradiated to the recording layer to heat it up to its melting point or higher. The recording layer to which was irradiated the recording power is melted, and then rapidly cooled, whereby amorphous record marks are formed on the layer. On the other hand, to erase the record marks, a relatively low-power (erasing-power) laser beam is irradiated to the recorded layer to heat it up to its crystallizing temperature or higher but lower than its melting point. The record marks to which was applied the erasing power are heated up to the crystallizing temperature or higher, and then gradually cooled, whereby the record marks are changed to crystalline. In that manner, therefore, overwriting is possible on phase change optical recording media by modulating the intensity of the single optical beam to be applied to the media.
Heretofore, the characteristics of phase change optical recording media have been judged as to whether or not the media could have a sufficient C/N ratio at the linear velocity at which they are operated and as to whether or not they could have a sufficient erasability. However, phase change optical recording media having a too high erasability would rather have poor characteristics. The high erasability of those media means that the recording layer of the media is easily crystallized. The easily-crystallizable recording layer is often problematic in that, while the trailing region of record marks is formed thereon, the leading region of the record marks having been formed is cooled due to the lateral heat diffusion, whereby the layer is recrystallized at that leading region. In other words, the leading region of the record marks formed on the recording layer is erased in that condition. This phenomenon is herein referred to as "self-erase". Of recording media having a too high erasability, the C/N ratio is lowered due to such self-erase. For these reasons, the erasability of optical recording media must be optimized.
In JP-A No. 9-7176, disclosed is a method for preventing self-erase that may occur in high-linear-velocity recording media when the media are used at a low linear velocity, in which the pulse pattern from the pulse division of the recording laser beam applied to a recording medium is optimized in accordance with the linear velocity for the medium.
However, as a result of our studies, we, the present inventors found the optimization in designing optical recording media for preventing the self-erase and for increasing the C/N ratio as high as possible brings about another problem. Concretely, the problem with the media having been so designed that they could have a highest C/N ratio at the linear velocity at which they are actually used is as follows: When the media on which record marks were formed are kept in a high-temperature condition at a temperature falling between 60 and 80.degree. C. or so, and are thereafter overwritten on the region of the record marks, the previously-formed record marks could not be satisfactorily erased. If the previously-formed record marks could not be erased, the newly-recorded signals could not be read out correctly, and jitter and error will increase greatly. The inside of media-driving units such as optical disc drives and others is heated at high temperatures of 60.degree. C. or higher while the units are driven, and therefore the media in the driving units are exposed to such high-temperature environment for a long period of time. In that situation, preventing the reduction in the erasability of recording media during high-temperature storage is an important theme.
A proposal to prevent the deterioration of the characteristics of phase change optical recording media during high-temperature storage was made, for example, in JP-A No. 5-159369, in which the dielectric layer formed on the substrate along with the recording layer was made of a composite composed of a chalcogen compound, an oxide and carbon (C). They say that, in the constitution disclosed in JP-A No. 5-159369, the inner stress to be formed inside the dielectric layer was reduced while the adhesiveness between the dielectric layer and the recording layer was increased, whereby the layer films were prevented from being peeled or cracked and the long-term reliability of the medium was ensured. However, in JP-A No. 5-159369, they are silent on the reduction in the erasability of the medium during high-temperature storage and even on the means of preventing it.